1. Field of the Invention
The present invention relates to an apparatus and a method for waste water treatment not generating excessive sludge.
2. Description of the Related Art
Heretofore, an activated sludge process has been employed generally for waste water treatment. In the process, the activated sludge is flocculated in a flocculation tank, while a part of the floc is returned to an aeration tank and the remaining part thereof, excessive sludge is drawn out of the system. With that, the process line can be constantly run under the condition of a BOD volume load falling between 0.3 and 0.8 kg/m3.day or so. On the other hand, carriers capable of carrying a high concentration of microorganisms are being developed for the process, and they enable a higher BOD volume load falling between 2 and 5 kg/m3.day applicable to the process line, and will realize down-sized aeration tanks for the process.
One problem with the conventional activated sludge process is that the BOD volume load to the process line is limited to the range falling between 0.3 and 0.8 kg/m3.day or so. In the process, therefore, large-size aeration tanks are inevitable. Another problem with it is that the excessive sludge must be drawn out of the system and must be treated in an additional line. In the activated sludge process, if waste water is treated under an elevated BOD volume load, its treatment is often insufficient. If so, in addition, the activated sludge will poorly flocculate and, as a result, continuously running the process line will be impossible. On the other hand, if complete oxidation is tried in the activated sludge tank without the excessive sludge being drawn out of the system, for which the sludge propagation rate shall be in proportion to the sludge self-oxidation rate, MLSS in the aeration tank will increase to a great extent, and, as a result, an extremely large-size activated sludge tank is inevitable in the process line. However, such a large-size tank is unfavorable. In addition, in such a case, the sludge will form fine particles and produces another problem in that it could not be separated from the treated water through spontaneous flocculation.
Compared with the activated sludge process, a process of water treatment with a carrier (this is hereinafter referred to as a carrier process) is advantageous in that a higher load is applicable to the process line and that down-sized aeration tanks are usable therein. In the carrier process, however, fine sludge particles that could not be separated from the treated water through spontaneous flocculation are formed, and they must be separated from it through forced flocculation with a flocculant. Therefore, the problem with the carrier process is that it requires an additional cost for the flocculant and that the floc must be further treated in an additional line.
The present invention has been made in consideration of the above-mentioned problems, and its object is to provide an apparatus and a method for waste water treatment which accept down-sized tanks for use therein not generating excessive sludge.
The apparatus for waste water treatment of the invention that solves the above-mentioned problems comprises an aeration tank where waste water is kept in contact with carrier particles in an aerobic condition, and a complete oxidation tank, and is characterized in that the liquid in the complete oxidation tank is filtered through a separation membrane and the resulting filtrate is discharged out.
In the complete oxidation tank, the waste water being treated is aerated under a low sludge load with the sludge propagation rate being kept in proportion to the sludge self-oxidation rate to thereby prevent the sludge increase. For this, the s-BOD sludge lo-ad in the complete oxidation tank is preferably at most 0.1 kg-BOD/kg-MLSS.day, more preferably at most 0.05 kg-BOD/kg-MLSS.day. In case where an ordinary activated sludge process line is run under such a low sludge load, the sludge therein will disperse and could not spontaneously flocculate, thereby producing a problem in that the sludge is difficult to separate from the treated water. Therefore, the waste water treatment apparatus of the invention is combined with a separation membrane through which the solid component (sludge) is separated from the treated water (filtered water). With that, it is possible to continuously run the waste water treatment apparatus of the invention not generating excessive sludge. As so mentioned hereinabove, in case where complete oxidation is tried in an ordinary activated sludge process, an extremely large-size activated sludge tank must be provided in the process line. Being different from such an ordinary activated sludge process, the waste water treatment apparatus of the invention is for a carrier process, in which, therefore, the aeration tank may be a compact one. In addition, since the majority of BOD can be removed from waste water in the aeration tank therein, the apparatus of the invention does not require a large-size complete oxidation tank. Even when the concentration of the activated sludge is low in a compact-size complete oxidation tank in the apparatus of the invention, the sludge propagation rate can be well kept in proportion to the sludge self-oxidation rate, and the apparatus does not require excessive sludge removal.